Vacuum actuated valves have long been used in the automotive industry for controlling the flow of various gasses, including exhaust gas recirculation (EGR). EGR systems use the valves for allowing exhaust gas from the exhaust manifold to flow into the intake manifold. Thus, the exhaust gas is recirculated through the engine. Controlling EGR flow is typically used for meeting regulated emissions and maximizing fuel economy. Poor EGR flow control can have detrimental effects, such as engine misfires, engine speed surging, and lost fuel economy.
In particular, good control at low exhaust gas recirculation flow rates is a challenge because of the relationship between the controllable signal sent to the valve and the actual flow through the valve. In one particular system, a vacuum signal is sent to a vacuum actuated valve. The vacuum actuated valve mechanically converts the vacuum signal to a valve pintle position through a spring and diaphragm assembly. The valve pintle then defines the flow area and thus the flow rate of exhaust gasses through the valve and into the engine. Because the pintle is designed to resist sticking and deposit coating, the flow to position characteristics are such that at low openings, a small change in valve position creates a large change in flow. In addition, at high openings, a large change in valve position creates a small change in flow. These relationships cause a high mechanical gain in the feedback control system at low openings. Further, when high gains are coupled with sensors that have delays, with low vacuum command resolution, or with low vacuum command repeatability, all of which are typical in EGR control systems, the result is poor control.
One method of eliminating the problems of poor control at low opening areas is to calibrate the command so that low EGR flow rates are not commanded. This eliminates the control problem by disallowing operating in a certain flow rate region. Another method of improving control of low flow rates would be to increase vacuum control resolution, vacuum control repeatability and sensor bandwidth. This would allow the control system to more accurately maintain a desired low flow rate despite the high gain between control vacuum and EGR flow rate.
The inventors herein have recognized numerous disadvantages with the above approaches. A disadvantage with removing commands of low flow rates is that this eliminates the benefit for which the EGR system is installed on an engine. In other words, there is both a fuel economy and regulated emission benefit for using low flow rates of EGR in certain engine operation regions. A disadvantage with increasing vacuum control resolution, vacuum control repeatability, or sensor bandwidth is that this generally adds cost and is unacceptable.